Peppermint (Western Oregon) Nutrient Management Guide

NUTRIENT MANAGEMENT EM 9018-E December 2010 Peppermint (Western Oregon) J.M. Hart, D.M. Sullivan, M.E. Mellbye, A.G. Hulting, N.W. Christensen, and G.A. Gingrich

he Pacific Northwest produces about 90 percent of the peppermint grown in the United States for oil, rootstock, and dried leaves. Dry peppermint leaves Tare used for tea and as a culinary herb. Peppermint oil is used to flavor chewing gum, toothpaste, mouthwashes, pharmaceuticals, and confectionary and aromatherapy products. Use for medicinal purposes is growing. Optimum oil quality and quantity are produced with ample daylight (usually more than 14 hours), warm day- time temperatures (85–95°F), and cool nighttime tempera- tures (55–60°F).

Gale Gingrich, © Oregon State University Peppermint (Mentha piperita) is typically produced on deep, well-drained soil in locations with plentiful good- quality irrigation water. Historically, peppermint was Summary grown on soil series such as Chehalis, Cloquato, and New- berg. These deep, well-drained sandy loam and silt loam Lime Soil pH between 5.5 and 6.0 is “bottomland” soils were formed in mixed alluvium depos- adequate for peppermint produc- ited by the ­Willamette River and its tributaries. Currently, tion. Before planting, apply lime peppermint is also grown on well-drained silt loam valley according to Table 4 when the soil floor terrace soils, such as the Amity, Malabon, Willamette, pH is below 6.0. In established and Woodburn soil series. See the sidebar “Peppermint stands, apply lime when the soil pH production in the U.S.” (page 2) for more information. Peppermint reproduces vegetatively via “runners” is below 5.5. See pages 9–10. (rhizomes and stolons), which originate from the main Nitrogen (N) Application of no more than 200 to rootstock in early to mid-June (Figure 1, page 2). Stolons 250 lb N/a is necessary for pep- are stems growing at or just above the soil surface. Rhi- permint under good irrigation water zomes are stems growing at or just below the soil surface. management. Supply approximately Both have fewer leaves than do vertical stems and can 175 lb N/a before mid-May. Do not produce new roots and shoots from buds. Other crops with apply N after late July. Monitor soil stolons include bentgrass, strawberries, and some clovers. nitrate-N on a few typical fields to Kentucky bluegrass and creeping red fescue have rhi- evaluate trends in soil N availability zomes. New root growth from stolons and rhizomes near for the crop (June) and late-season the soil surface creates the impression that peppermint is soil nitrate-N accumulation (July a shallow-rooted crop. In fact, peppermint roots are easily and August). See pages 5–7. found 2 feet below the surface. Phosphorus (P) When soil test P is below 40 ppm, The “aggressive” or “invasive” reproductive nature apply P according to Table 5. of peppermint is exploited for establishment. Stands are See pages 10–11. established by planting stolons or rhizomes in rows. The Potassium (K) When soil test K is below 200 ppm, new plants spread quickly throughout the field; by the apply K according to Table 6. second year, rows are no longer distinguishable. Stands See page 11. Sulfur (S) Apply 20 to 30 lb S/a in early spring with the first application of N. J.M. Hart, Extension soil scientist; D.M. Sullivan, Extension soil scientist; M.E. Mellbye, area Extension agronomist; A.G. Hulting, Extension weed management specialist; N.W. Christensen, profes- 1 sor of soil science emeritus; and G.A. Gingrich, area Extension agronomist emeritus; all of Oregon State University. remain in production for as few as 3 or as many as 15 years. The average Willamette Valley peppermint field produces oil for 5 to 7 years. The oil is stored in glands on the leaves. As plants approach bloom, oil concentration increases, and the lower leaves drop from the plant. Oil concentration reaches its peak during bloom. Cutting is recommended at early bloom development stage to minimize leaf loss and maximize oil concentration and quality. Peppermint is a challenging crop to grow. Timely water and fertilizer applications are essential for optimum yields and quality oil. Nitrogen (N) is the most yield-limiting fertilizer nutrient. Liming to increase soil pH is necessary, Figure 1.—Peppermint is a perennial aromatic herb with erect, as is the addition of phosphorus (P), potassium (K), and square, branched stems. Leaves are arranged on the stem in opposite sulfur (S). pairs. Illustration: Kathy Merrifield This guide provides nutrient and lime recommendations for first-year and subsequent-year single-cut peppermint Double-cut peppermint crops in Clackamas, Benton, Lane, Linn, Marion, and Polk counties. For peppermint harvested twice in a grow- A few western Oregon peppermint fields are ing season, see the sidebar “Double-cut peppermint.” harvested twice in a growing season (June and Sep- The average peppermint oil yield in the Willamette tember). This practice is known as “double-cutting.” Valley is between 90 and 100 lb/a. Recommendations in Cutting twice in a season can increase oil yield by this guide are adequate to routinely produce a peppermint 30 to 40 percent, but oil quality will differ from that oil yield greater than 100 lb/a, assuming that disease-free of peppermint harvested during early bloom. For rootstock of an appropriate variety is planted on suitable information about oil quality, see the sidebar “Pep- soils free of verticillium wilt. permint oil quality” (page 8). Recommendations in this guide are based on research No research has been performed for double-cut conducted during the past 50 years on both large plots in peppermint in the Willamette Valley. Nitrogen rate is grower fields and small plots at Oregon State University likely to be the primary nutrient management adjust- research facilities in the Willamette Valley. See Appen- ment needed for this harvest system. At the first dix A (page 14) for a list of research projects. harvest (mid- to late June), 150 to 175 lb N/a has been accumulated by the crop. If the second harvest produces a similar hay yield, then two applications of 150 to 175 lb N/a will be needed.

Peppermint production in the U.S.

Peppermint production in the United States began acreage had increased to 15,900. Peppermint produc- in 1812, when stolons from England were planted in tion reached a peak of 50,000 acres in 1996. About Massachusetts. By 1835, production had moved to the half of Oregon’s mint production was in Marion, Linn, midwestern U.S. The soils and climate of Michigan and Lane, Benton, and Polk counties. Indiana were well suited to peppermint culture. These By 2008, harvested acreage in Oregon had states became the major peppermint-producing area by decreased to 21,000 acres. The Willamette Valley 1920, with production peaking in 1930. continues to grow approximately one-half of Oregon’s Verticillium wilt led to a decline in production in the peppermint crop. Peppermint is also produced in Midwest and an increase in acreage in Washington and Columbia County, in the irrigated areas of Morrow Oregon. After 1950, the Pacific Northwest became the and Umatilla counties, in the central Oregon counties largest peppermint- and spearmint-producing area in of Jefferson, Crook, and Deschutes, and in the Grande North America. Currently, the northwestern states of Ronde Valley of Union County. Montana, Idaho, California, Washington, and Oregon For Willamette Valley farms with irrigation, pep- produce between 85 and 90 percent of the peppermint permint oil is a high-value crop. During the peak years oil in the U.S. Oregon’s production is about one-third of of production, 25 to 40 percent of irrigated cropland in the national total. the Valley was planted to peppermint. Peppermint pest In Oregon, peppermint production began in 1908 near and weed control practices make it an ideal rotation Eugene. By 1939, 2,200 acres were harvested; by 1960, crop with grass seed, wheat, and sugar beets for seed.

2 Healthy plants with adequate root systems are required canadensis), and prickly lettuce (Lactuca serriola). See to obtain the greatest return from your fertilizer invest- Appendix C (page 16) for information about the influence ment. The nutrient recommendations in this guide assume of weeds on peppermint oil quality. Specific weed man- adequate control of weeds, insects, and diseases. Lack agement recommendations are found in the mint chapter of pest control cannot be overcome by the addition of of the PNW Weed Management Handbook and in OSU nutrients. Extension publication EM 8774, Weed Management in Common diseases of peppermint are verticillium wilt Mint. (Verticillium dahliae), powdery mildew (Erysiphe cicho- Numerous insects and invertebrate pests attack pepper- racearum), and leaf rust (Puccinia menthae). Planting mint, including the two-spotted spider mite (Tetranychus disease-free roots (certified wilt-free if possible) in fields urticae), root borers (Fumibotys fumalis), several cut- free of verticillium wilt helps assure a productive, healthy worms and armyworms, alfalfa looper (Autographa cali- stand for several years. See the sidebar “Variety and verti- fornica), cabbage looper (Trichoplusia ni), grasshoppers cillium wilt” for more information. (Camnula pellucida), multiple slug species, symphylans Nematodes are serious soil pests that can interact with (Scutigerella immaculata), and flea beetles Longitarsus( verticillium wilt to severely weaken mint stands. Sample waterhousei). For more information about insect manage- fields to assay nematodes and control nematodes prior to ment in peppermint, see the PNW Insect Management planting. Handbook or PNW 182, A Guide to Peppermint Insect Control of weeds is essential for reducing competi- and Mite Identification. tion with the crop and ensuring oil quality. Common weeds in peppermint fields include annual and perennial grasses, as well as broadleaf weeds such as common Understanding peppermint groundsel (Senecio vulgaris), field bindweed (Convolvu- growth lus arvensis), Canada thistle (Cirsium arvense), common The growth pattern for crops such as wheat and grass lambsquarters (Chenopodium album), annual sowthistle grown for seed resembles an elongated “S” with three (Sonchus oleraceus), pigweed species (Amaranthus spp.), segments or “phases.” The first phase is the “tail” of the nightshade species (Solanum spp.), marestail (Conyza “S” as growth slowly begins. The second phase is rapid

Variety and verticillium wilt

Profitable peppermint production and stand longev- ity require that disease-free rootstock of an appropriate variety be planted in fields free of verticillium wilt. The traditional variety, Black Mitcham (Figure 2), is susceptible to verticillium wilt. When a wilt-free field is not assured (due to a history of mint production), an alternative variety such as Redefined Murray will pro- duce more peppermint hay and have less verticillium wilt infection than Black Mitcham (Table 1).

Figure 2. Newer varieties (background) produce higher yields in verticillium-infested fields than does the traditional variety Black Mitcham (weedy plot in foreground). Photo: Mark Mellbye

Table 1.—Influence of verticillium wilt on Willamette Valley peppermint production, by variety and field history.a Noninfested soil Soil infested with verticillium wilt Wilt Ground Wet hay Wilt Ground Wet hay infection cover yield infection cover yield Variety (%) (%) (lb/a) (%) (%) (lb/a) Black Mitcham 0.4 99 30,100 67 50 5,300 Redefined Murray 0.1 100 37,500 20 95 19,900 M-83-7 0.1 100 38,900 27 95 19,000 aData collected during the fourth production year, 1999. Fields were part of a variety trial conducted at the Botany and Plant Pathology Farm, Corvallis, OR, 1996–2000.

3 growth, with the crop sometimes seeming to change daily. lower leaves. Sometimes, peppermint will lodge, “turn,” The third phase is characterized by flower and seed devel- and grow new stem and leaf material. opment, in addition to movement of nutrients from leaves Residue remaining in the field after leaves and stems into the developing seed or fruit. Little nutrient accumula- are removed for oil distillation represents a signifi- tion occurs during Phase 3. When both nutrient accumula- cant portion of peppermint dry matter production. This tion and growth stop, the line becomes flat. material is between 1,000 and 1,500 lb/a and contains Peppermint grown for oil production does not fit this 40 lb N/a, 4 lb P/a, and 15 lb K/a. Peppermint roots from three-phase growth model, as the crop is harvested during the top 8 inches of soil weigh about 2,500 lb/a and contain bloom. Thus, the peppermint growth pattern has only two 50 lb N/a, 10 lb P/a, and 30 lb K/a. segments: After harvest, peppermint is irrigated and begins to • Slow early growth from late February to late April or grow new leaves and stems. This postharvest growth pro- early May duces 1,000 to 2,000 lb dry matter/a, which contains 40 to 50 lb N/a, 4 to 5 lb P/a, and 30 to 40 lb K/a. • Rapid linear development from early or mid-May to Oil weight represents from 0.5 to 2 percent of dry mat- harvest ter. Oil yield is determined by the quantity of leaves on Figure 3 illustrates the linear, steady growth of late the plant. Oil concentration in the upper 15 inches of a spring and summer. Total harvested dry biomass ranges plant (stems and leaves) is 1.3 percent, while oil concen- from 8,000 to 10,000 lb/a. The maximum biomass accu- tration from the entire plant is slightly less than 1 percent. mulation rate of 75 to 125 lb dry matter/a/day occurs in late June to mid-July (Figure 4). Peppermint hay at harvest contains 175 to 225 lb N/a Collecting soil samples (Figure 3). The maximum N uptake rate of 2 to 3 lb/a/day Before planting peppermint, collect soil samples to occurs in May to early June, 30 to 40 days before peak plow depth. A sample should contain a minimum of biomass accumulation. By the end of June, the N uptake 20 cores and represent no more than 40 acres. Request rate falls to less than 0.5 lb/a/day (Figure 4). sample analyses for pH, SMP buffer (preplant only), phos- The rapid growth of peppermint during May and early phorus, potassium, calcium, and magnesium. June creates sufficient leaves for a full canopy, which Nutrient stratification commonly occurs in peppermint intercepts all sunlight. From the time the canopy “closes” fields from top-dress fertilizer application without tillage. until harvest, new leaf growth is accompanied by loss of For this reason, we recommend collecting soil samples from two depths on established fields: (1) the surface 2 inches of soil, and (2) the 2- to 6-inch layer of soil. 250 12,000 Additional information on nutrient and soil pH stratifica- tion can be found in OSU Extension publication EM 9014-E, 200 9,000 “Evaluating Soil Nutrients and pH by Depth in Situations of Limited or No Tillage in Western Oregon.” See also the 150 6,000 sidebar “Sampling depth for P and K” (page 10). 100 If peppermint is not growing well in some parts of a Nitrogen 3,000 field, collect plant samples from both “good” and “bad”

50 Biomass (lb/a) N uptake (lb/a) Biomass areas. 0 0 5/1 5/31 6/30 7/30 Date Figure 3.—Cumulative nitrogen uptake and biomass accumulation for peppermint.

4 120

3 90

Nitrogen 2 60 Biomass 1 30 N uptake (lb/a/day) Biomass (lb/a/day)

0 0 5/1 5/31 6/30 7/30 Date Figure 5.—Soil sampling in an established peppermint field. Photo: Figure 4.—Rate of nitrogen uptake and biomass accumulation for Mark Mellbye peppermint.

4 250 Nitrogen (N) Peppermint hay at harvest contains 175 to 225 lb N/a. 200 1997 Nitrogen is the nutrient most likely to limit growth and 1996 yield of peppermint. Application of 200 to 250 lb N/a is adequate for peppermint that is not excessively irrigated 150 (Figure 6). Nitrogen can be supplied through irrigation water or to the soil early in the growing season. 100 Growers typically apply some N in late March or early April. Research has not been performed to confirm the N uptake (lb/a) 50 need for N application at this time. Nitrogen rate and tim- ing research in 1970 and 1971 indicated that oil yield was 0 maximized with application of approximately 175 lb N/a 5/1 5/31 6/30 7/30 8/29 before mid-May. This recommendation was confirmed Date by research in 1996 and 1997. N applied after late July or Figure 8.—Cumulative N uptake in a Lane County peppermint early August is likely to remain in the soil and be lost to field, 1996 and 1997. leaching with winter rain. Yearly variation in early-season growth controls N accumulation by peppermint. As shown in Figure 7, early- Nitrogen management summary season peppermint biomass (hay) production from the same field was substantially less in 1996 (a cool spring) • Apply N fertilizer at a rate that produces ade- than in 1997 (a warm spring). Figure 8 shows N uptake quate yield and minimizes nitrate accumulation from this field in 1996 and 1997. By early June 1996, the in soil in August. amount of N in the peppermint crop was about 50 lb/a. In 1997, the N in the crop was double this amount, 100 lb/a. • Apply no more than 200 to 250 lb N/a/yr. This Adjust N timing based on spring weather conditions. application rate supplies adequate N under cur- Nitrogen management recommendations are summa- rent management practices. rized in the sidebar “Nitrogen management summary.” • Irrigate to meet crop evaporative demand. Use Additional information is provided in the sidebars “Nitro- irrigation scheduling and monitoring tools gen leaching below the root zone,” “Nitrogen monitor- to maintain adequate, but not excessive, soil ing,”90 and “Peppermint oil quality” (pages 6–8). ­moisture. • Target N management and irrigation manage- 75 ment toward productive portions of the field. Crop growth on gravel bars may be poor when 60 the rest of the field has optimum N and irrigation water applied.

Oil yield (lb/a) 45 • Maintain the sprinkler irrigation system to apply water uniformly. 30 • Schedule N fertilizer applications to precede 0 50 100 150 200 250 300 crop need. N rate (lb/a) • Apply N fertilizer at two or more application Figure 6.—Peppermint oil yield and nitrogen rate from a Benton dates (split application). Split N application is County peppermint field, 1971. of greatest benefit on sandy soils that have low water-holding capacity. 12,000 • Eliminate late-season N fertilizer applications. Nitrogen fertilizer applications 3 weeks to a 9,000 1996 month before harvest are not effective in increas- 1997 ing crop yield or oil quality. 6,000 • Do not apply slow- or controlled-release N fertilizers during late June or early July, because some of the N will be released too late for crop 3,000 uptake.

Biomass/peppermint hay (lb/a) • Monitor trends in soil nitrate-N on a few typical 0 fields to evaluate soil N availability for the crop 4/30 5/30 6/29 7/29 8/28 Date (June) and excess soil nitrate-N accumulation in Figure 7.—Peppermint hay or biomass production in a Lane late season (July and August). See page 7. County peppermint field, 1996 and 1997. 5 Nitrogen leaching below the root zone

Small-plot lysimeters and field-scale, replicated • Nitrate-N concentration in leachate is not measured, research plots were used to estimate N leaching. Both but rather is estimated from weather data and soil approaches reached the conclusion that with existing water-holding capacity. management practices, typical nitrate-N leaching losses Large-scale N rate experiments were conducted in approach 100 lb N/a/yr. 1996 and 1997 to compare applications of 230 lb N/a This research demonstrated that opportunities exist and 320 lb N/a on a Lane County and Marion County to improve crop utilization of N, thereby decreasing the farm. Fertilizer application was split into three or four amount of N leached. Crop efficiency at using N is a applications, with most of the N applied by mid-June. function of N fertilizer management practices and the The high N rate treatment received the extra N fertilizer distribution and timing of irrigation water application. (90 lb N/a) in mid-June. Oil yield (average 97 lb/a) and crop N uptake (average 202 lb/a) were not affected by Monitoring with small-plot lysimeters N fertilizer rate (4 site-years). The lysimeter is a catch basin placed under an undis- Postharvest soil nitrate-N (0- to 5-foot depth) turbed soil profile. Water drains from the soil profile increased when the N fertilizer rate increased from 230 to into the lysimeter, providing an accurate estimate of the 320 lb/a (Figure 9). The top 2 feet of soil contained most N quantity leached per unit area, as well as the nitrate of the unused N. Most of the nitrate-N present in the concentration in the leachate. Measurement of a small entire 0- to 5-foot sample was gone by the next spring area (10 square feet) is the major limitation of lysimeters. (March), with leaching the most likely mechanism for Nitrate leaching from five farmer-managed Lane nitrate loss. Soil nitrate-N decreased by more than 100 lb County peppermint fields receiving an average of N/a between late August and early April. 250 lb N/a/yr was monitored (1993–1998). Leachate volume at a depth of 3 to 4 feet, nitrate concentration, and lysimeter water collection efficiency were used to Conclusions estimate annual N loss via leaching. • Oil yields did not benefit from N fertilizer rates in An average of 82 lb N/a/yr was leached, with an aver- excess of 230 lb N/a. age leachate nitrate-N concentration of 12 mg/L (ppm). • Over-winter nitrate-N loss exceeded 100 lb N/a/yr Individual field averages for N loss ranged from 40 to using moderate N fertilizer rates (230 lb N/a). 120 lb N/a/yr. Field averages for leachate-N concentra- • Monitoring of nitrate-N in the top foot of soil was a tion were 6, 8, 11, 17, and 23 ppm (mg/L). good indicator of soil profile nitrate-N. In late sum- This study demonstrated that nitrate-N leaching losses mer, 45 percent of the nitrate in a 5-foot soil profile under peppermint can be a concern for groundwater was present in the top foot (0 to 12 inches), 20 per- quality. Nitrate-N concentrations in excess of 10 mg/L cent in the second foot (12 to 24 inches), and only are considered unsafe in drinking water. 35 percent in the lower profile (24 to 60 inches). • Monitoring for soil ammonium-N increased cost, but Monitoring agronomic field trials did not provide useful information. Replicated plots that are large enough to be fertilized and harvested using grower equipment (1.5 acre) offer Topsoil (0–24 in) an opportunity to compare the effects of fertilization Subsoil (24–60 in) practices on oil yield, crop N uptake, and soil nitrate-N concentrations. This method can provide a field-scale estimate meaningful to growers. Soil samples for nitrate-N analysis are collected in 1-foot depth increments to a depth of 5 feet. Soil profile High N rate nitrate-N is measured in the spring and fall to determine soil nitrate-N concentrations. Loss of soil nitrate-N over the winter is attributed to leaching below the root zone. This method has the following limitations: Moderate N rate • Samples are usually taken only when tractor-mounted sampling equipment can be used without damaging the crop, usually in the spring and after harvest. • N leaching loss is calculated as the difference Figure 9.—Effect of nitrogen fertilizer rate on soil nitrate-N between fall and spring concentrations. This method remaining in late summer (August 28) and the following spring is less reliable than a direct measurement. (April 2). High N fertilizer rate = 320 lb N/a; moderate N fertil- izer rate = 230 lb N/a. Source: Christensen et al. (2003)

6 Nitrogen monitoring

The goal of N management is to support crop needs, Stem nitrate-N: Choose stems that receive ade- while minimizing the amount of soil nitrate-N remain- quate sunlight and have typical internode length. Do ing after harvest. Soil and plant tissue monitoring can not sample shaded stems from deep within the plant provide useful information about N fertilizer practices. canopy (near the ground) or upright, purplish stems Early-season soil sampling or tissue monitoring (May that are budding or flowering on top of the canopy. to mid-June) is generally not helpful in predicting N Collect a composite sample of 30 stems by cutting fertilizer need. Monitoring from mid-June to harvest stems 6 inches below the shoot tip. It takes 30 stems to can assist in (1) determining the need for additional N get sufficient dry matter for lab analysis because stems application in June or July and (2) developing a database are mostly water. Remove the leaves from stems and to assist in evaluating your N fertilizer management. discard. Send the top 6 inches of stems (minus leaves) Following harvest, review N monitoring data and crop to the lab. Keep the sample cool and ship to the lab via production records to evaluate your N management pro- next-day delivery. gram and identify opportunities for improvement. Research was conducted in Lane County produc- Rather than sampling many fields a single time, we tion fields to evaluate the effects of sampling protocol recommend choosing a few “typical” fields to monitor on stem nitrate-N values (Smesrud and Selker 1998). weekly or biweekly from June through harvest. Choose Results showed that the sampling protocol outlined 1-acre areas in productive portions of the field. Select above should provide a reproducible stem nitrate value areas with typical management practices, avoiding within 10 to 15 percent of the true mean value. Sam- gravel bars and other atypical areas. You can also collect pling time of day did not have a major effect on results. soil moisture data from the same areas for irrigation scheduling. Interpretive values We recommend monitoring both soil and stem nitrate, Trends in soil and stem nitrate values during the as these two measurements reflect complementary growing season are of greater value for evaluating aspects of N management. N management than are single test values. Use the The soil nitrate-N test measures N currently available information below as a starting point for interpreting for plant growth. Trends in soil nitrate indicate whether monitoring data. plant-available N is being lost (via plant uptake or leach- Soil nitrate-N: Soil test nitrate-N is usually ing) or accumulated (from fertilizer, mineralization of expressed in lb/a. A soil containing 20 ppm soil organic matter, or irrigation water). nitrate-N (0- to 1-foot depth) contains approximately The stem nitrate test is a measurement of nitrate as it 70 lb nitrate-N/a. During June, when soil nitrate-N enters the plant but before it reaches the leaves. There- ­values in the 0- to 1-foot depth fall below 20 ppm, fore, it is a direct assessment of N supply as “seen” by plants may respond to N application. the plant. Monitoring of other nutrients via stem sam- Research has not been conducted to determine the pling may also help assess plant nutrition. minimum level of nitrate-N needed in July and August Although both tests provide useful information, we for optimum crop growth and oil yield. Because the consider soil nitrate-N to be a better indicator of crop N N uptake rate declines in July, soil nitrate-N can be status than stem nitrate-N. Soil nitrate-N is a direct mea- lower than 20 ppm at harvest without compromising surement of current plant-available N supply. The stem oil yield. nitrate test indirectly measures soil N effects on plant Stem nitrate-N: Replicated field trials to identify tissue N concentration. As an indirect measurement, deficient, adequate, and excess stem nitrate-N values stem nitrate test results can be confounded by envi- have not been conducted in the Willamette Valley. ronmental factors that affect plant metabolism. When Guidelines from other growing regions may not be limited resources are available to support monitoring helpful under local conditions. Stem nitrate-N val- activities, soil nitrate-N is the preferred measurement. ues greater than 6,000 to 8,000 ppm (mg/kg) likely indicate adequate N supply in early season. Sufficient Sampling method values may decline to 2,000 to 4,000 ppm near harvest. Soil nitrate-N: Collect at least 15 cores to a depth When N is applied frequently via sprinkler irriga- of 1 foot using a soil probe. Combine cores in a sturdy tion, and plants are growing rapidly, stem nitrate-N plastic or plastic-lined bag and submit the composite may not increase following N application. When most sample to the lab for analysis. Keep the sample cool and of the N fertilizer is applied early (June), stem ship to the lab via next-day delivery. If sample results nitrate-N values may be high in June and decline rap- are not needed immediately, samples can be frozen and idly as the crop matures. Both trends in stem nitrate-N shipped later. during the growing season may indicate sufficient N for peppermint grown in the Willamette Valley.

7 Peppermint oil quality

A peppermint plant synthesizes oil in glands on its Nitrogen and maturity leaves. Oil quality is determined by the balance of sev- The influence of N rate and peppermint maturity on eral oil constituents. Many factors influence the com- oil composition was measured near Corvallis in 1970 position of distilled oil, including stage of maturity at and 1971. N rates were 50 to 300 lb/a, and pepper- harvest; environmental conditions throughout the grow- mint was harvested at bud, 50 percent bloom, or full ing season and at harvest; crop management, including bloom. Results are shown in Table 3 and Appendix B nutrient source and time of application; postharvest han- (page 15). dling techniques; and the presence of weeds in the field. Based on results of this research, we recommend The effects of N and S application and crop maturity are application of 200 to 250 lb N/a and harvest at 10 per- discussed in this section. cent bloom. This combination of N rate and harvest The quality of mint oil is determined by labora- time should yield an acceptable balance of oil constitu- tory chemical analysis and by organoleptic evaluation ents. Since not all fields can be cut at one time, typical (aroma testing by trained oil quality analysts). As shown harvest for a grower would probably begin with less in Table 2, superior peppermint oil contains high quanti- than 10 percent bloom and be completed with slightly ties of menthol, moderate amounts of menthone, and greater than 10 percent bloom. low levels of pulegone and menthofuran. Menthol is the component that creates peppermint’s characteristic cool, Effects of S application refreshing sensation. S fertilizer is generally applied to the soil early in Nighttime temperatures below 55 to 60°F during the the growing season (March–April) in the form of growing season favor production of menthone and men- sulfate-S (SO4) at a rate of 20 to 30 lb S/a. In addition thol. Warm nighttime temperatures allow conversion of to the sulfate form, S is commonly sold in the elemen- these compounds to pulegone and menthofuran, thus tal form. Elemental S should not be used as a source reducing oil quality. of current-season S for peppermint or most other crops because it is not available for crop uptake until bacteria Table 2.—Acceptable composition of peppermint oil from oxidize it to the sulfate form. This change usually does a single-cut production system in western Oregon. not occur rapidly enough to supply plant-available sulfate-S. Range a Elemental S is sometimes applied to peppermint as Oil component (%) a fungicide. This use of S shortly before harvest can Limonene Below 1.8 reduce oil quality. The effects of sulfate-S and elemen- Eucalyptol 4.0–5.5 tal S on oil quality were compared in commercial Menthone 15–25 field-grown peppermint. These studies confirmed that Menthol 39–45 sulfate-S does not influence peppermint oil quality. Menthyl acetate 5–6 However, elemental S, if improperly applied, can Menthofuran Below 2.5 decrease peppermint oil quality. To reduce potential oil Pulegone Below 2.0 quality degradation, apply the lowest effective rate of Piperitone 0.4–0.9 elemental S needed for disease control in conjunction aDetermined by gas chromatography. with the longest possible preharvest interval. Consult the Oregon State University online PNW Plant Disease Management Handbook for current recommendations.

Table 3.—Peppermint oil quality change with maturity and N rate. Influence Suggested N rate Influence Peppermint oil component of N rate (lb/a) of maturity Terpene hydrocarbons Little NA Decrease Menthone Increase 200–250 Decrease Menthol Decrease 200–250 Increase Menthyl acetate Little NA Increase Menthofuran Decrease 200 Increase Oil yield Increase 200–250 Increase Oil yield and quality — 200–250 —

8 Topdress lime application Lime, calcium, and magnesium Ammoniacal N fertilizers (e.g., urea) readily reduce soil pH in the surface layer on the lighter textured soils Importance of soil pH where mint is grown. When ammoniacal N is topdressed, Soil pH indicates the chemical condition roots will surface soil pH decreases about 0.1 pH unit/yr for every experience. As soil pH decreases (soil becomes more 100 lb N/a applied. Since more than 200 lb N/a is typi- acidic), solubility of iron, zinc, manganese, and aluminum cally applied annually to peppermint, the soil pH could increases. If soil is too acidic, the concentration of alu- decrease 0.5 unit in as few as 3 years, especially on a minum can reach levels that inhibit root growth. Soil pH sandy soil such as the Newberg series. analysis indicates whether soils are too acidic and whether This reduction in soil pH could lead to the need for lime is needed to raise soil pH. Lime rate is estimated lime topdressing, but topdressed lime creates soil pH with another test, the SMP buffer test. stratification, since lime has limited mobility. Lime A soil pH between 5.5 and 6.0 is adequate for pep- increases soil pH only in the surface layer of soil permint production. The most important preplant nutrient (­Figure 10). If soil pH falls below 5.0, topdressed lime management activity is to ensure a soil pH of at least 6.0. will produce a suitable soil pH only at the surface. Soil This will ensure that soil pH remains adequate for several pH even slightly below the surface can remain too low to years. maintain adequate crop growth. The stratification in soil pH from N fertilizer or lime Preplant lime application application is reason to collect soil samples from two Sample and test soil for pH, SMP buffer lime require- depths, 0 to 2 inches and 2 to 6 inches. Apply lime via ment, and calcium (Ca) well before planting, because lime topdressing when the soil pH in the surface 2 inches is 5.5 should be mixed into the field before planting. or below. Monitor soil pH in the 2- to 6-inch depth. If it Soil pH naturally varies through the year by as much falls below 5.5, peppermint oil yield will likely decline. as 0.3 to 0.5 unit. It is lowest in late August and Septem- The only solution to increasing soil pH below the surface ber, before the fall rains begin, and highest in February 2 inches is to mix lime into the soil. Incorporating lime or March, when the soil is wettest. Sample soils for pH into the soil may necessitate removing the peppermint at the same time each year to avoid confusion caused by stand. seasonal fluctuation. Soil pH increase following topdressed lime application The Shoemaker-McLean-Pratt (SMP) buffer or lime depends on lime rate, soil type, crop, and management. requirement test (LR) estimates the amount of lime Normally, topdress lime application rates are 1 or 2 t/a. In needed to raise soil pH. Preplant lime application rates some situations, this lime rate increases soil pH to a depth can be estimated from Table 4. A lime application is of 1 or possibly 2 inches in the first year after application. effective for several years. See OSU Extension Service Below this depth, soil pH is unlikely to change. Changes publication FS 52-E, Fertilizer and Lime Materials, for in soil pH after the first year are uncommon. more information about liming materials. When more than 3 to 4 t lime/a is recommended, mix in Soil pH half the lime before or during plowing and incorporate the 4 5 6 7 remainder with tillage after plowing. This strategy also can be used with lower lime application rates. 0

Table 4.—Preplant lime rate recommendations for western 1 Oregon using the SMP buffer. b SMP buffer Apply this amount of lime a 2 test result (t/a) Below 5.2 5–6 5.2–5.6 4–5 3 5.6–5.9 3–4 5.9–6.1 2–3 4 Poor Growthgrowth

6.1–6.5 1–2 Soil depth (inches) Over 6.5 None Good Growthgrowth aLime recommendations are based on the SMP buffer test only. If 5 other buffer tests are used, recommendations may differ. Note that the SMP buffer does not equal soil pH. Liming rates cannot be determined based solely on soil pH. 6 bLime rate is based on 100-score lime. Figure 10.—Soil pH at varying depths from areas of good and poor growth in a Benton County peppermint field.

9 On established fields, topdress lime as soon after potassium-magnesium sulfate (approximate analysis harvest as possible so that it can react before spring N 22 percent K2O, 11 percent Mg, 22 percent S), marketed fertilizer is applied. Surface application of lime can cause as Sul-Po-Mag or K-Mag. volatilization losses from topdressed ammoniacal N mate- rials such as urea. The chance of volatile N loss is greatest when urea remains on the surface of a recently limed field Phosphorus (P) for more than a day. The chance of N loss through vola- Between 30 and 40 lb P/a is removed in peppermint tilization is reduced when lime reacts through the winter hay. Unlike N, limited P application research has been or through several applications of irrigation water. During conducted. In the few Oregon and Washington research the season following a topdress lime application, irrigat- trials using P rates, a benefit from P application was rarely ing immediately following N application is prudent. measured. A 1967 survey of 60 fields found “high” tissue P with no apparent relationship between P concentration and oil yield. Calcium (Ca) and magnesium (Mg) P application did not increase oil yield in Montana In addition to soil pH, review extractable Ca before fields with low soil test P (Olsen or bicarbonate extract- planting peppermint. If extractable Ca is less than able P of 5 ppm). These results led to speculation that 8 meq/100 g soil (1,600 ppm), add 1 t lime/a even if soil established mint fields become infected with mycorrhizae, pH is above 5.5. a fungus that aids plants in P uptake. Peppermint oil yield increase from application of Mg Determine P need from soil test results and use Table 5 has not been documented in western Oregon. Nonetheless, (page 11) to estimate P rate. Apply P in fall to established we advise adding Mg if soil test Mg is below 120 ppm stands when soil test P is below 30 ppm. When soil test P or 1 meq/100 g soil. Magnesium can be supplied from is between 30 and 40 ppm or for maintenance application, 1 t dolomitic lime/a or by applying 30 to 50 lb Mg/a from P can be applied in either spring or fall. See the sidebar “Sampling depth for P and K” for more information.

Sampling depth for P and K

Comparisons between soil test values from the 160 surface 2 inches and the soil below can help you make cost-effective nutrient management decisions. The 120 examples here use a sample from the 2- to 10-inch

(ppm) surface 2 inches

depth. This depth is not recommended; it is used only P as an example. 80 In Figure 11, P soil test values in the 0- to 2-inch 10-inch depth 40

sample increased as stand age increased. In the 2- to Soil test 10-inch sample, however, they remained constant. 2- to 10-inch depth Based on the P soil test value from the 2- to 10-inch 0 sample, a P application would be recommended. How- 1976 1979 1982 1984 1987 1990 1993 1995 ever, the sample from the 0- to 2-inch depth shows that Date additional P is not needed, since peppermint roots read- Figure 11.—Soil test phosphorus for three soil sample depths ily use P from the surface 2 inches of soil. By sampling during the life of a peppermint stand, Polk County, OR. soil from multiple depths, this grower could reduce or 800 eliminate P fertilizer application. Figure 12 shows declining K soil test values in all 600 sample depths. Two important points are seen: surface 2 inches • Peppermint uses nutrients from at least the surface foot of soil. 400 • A single sample from the surface 2 inches is not 10-inch depth adequate for understanding soil test K depletion. 200 Soil test K (ppm) Note: The peppermint stand represented in 2- to 10-inch depth ­Figures 11 and 12 was about twice the age of most 0 fields. The age, fertilizer application history, and con- 1976 1979 1982 1984 1987 1990 1993 1995 sistent collection of soil samples provide an excellent Date Figure 12.—Soil test potassium for three soil sample depths example of soil test changes that can occur in pepper- ­during the life of a peppermint stand, Polk County, OR. mint fields.

10 Table 5.—Fertilizer phosphorus (P) rate recommendations for western Oregon using the Bray P1 soil test. Use this table for preplant soil sample from 0 to 6 inches and 0 to 2 inches in established stands. a Soil test P Apply this amount of P2O5 (ppm) (lb/a) Below 20 100–150 20–30 60–100 30–40 0–6 Over 40 None aBray P1 soil test method.

Potassium (K) Figure 13.—Mint hay harvest removes large quantities of potassium Potassium is the second most likely nutrient (after N) from the field, potentially depleting soil test K. Photo: Gale Gingrich to limit peppermint hay and oil yield. Harvest of 4 to 5 t/a of peppermint hay removes 250 to 350 lb K/a (Figure 13). Many soils on which peppermint is produced are sandy Sulfur (S) and relatively low in organic matter. The combination of Crops grown in western Oregon require addition of sandy, low-organic-matter soil and K use by peppermint S, usually 15 to 30 lb/a. The amount and frequency of can rapidly deplete soil test K. application depends on soil S supply and on crop removal. Conversely, annual topdress of K fertilizer will cre- Sulfur content of peppermint hay varies and is influenced ate stratification in soil test values. The surface 2 inches by N fertilizer application, S application, and the amount of soil can easily have a soil test K value double the of biomass produced. From 1995 through 1997, a range of amount found in a sample from 2 to 6 inches. See the 6 to 40 lb S/a was measured in hay harvested from seven sidebar “Sampling depth for P and K” (page 10) for more peppermint fields, with an average of 20 lb S/a. ­information. Apply 20 to 30 lb S/a in early spring with the first The stratification from K fertilizer application is reason application of N. Plants use S as sulfate (SO4-S). Fertilizer to collect soil samples from two depths, 0 to 2 inches and materials such as ammonium sulfate, ammonium phos- 2 to 6 inches. Apply K to soil with less than 200 ppm phate sulfate (16-20-0-14S), Sul-po-mag, urea-sul, and ammonium extractable K in the surface 2 inches of soil. potassium sulfate (0-0-50) supply sulfate-S. See the side- Monitor soil test K in the 2- to 6-inch depth. When it bar “Peppermint oil quality” (page 8) for more information declines, increase the annual K application rate. When the about how various forms of S influence oil quality. soil test value from this sampling depth is above 200 ppm and increasing, the annual K rate can be decreased. If the soil test K value from this depth does not change, K sup- Micronutrients—Zinc (Zn), ply is adequate and your current fertilizer program can be copper (Cu), boron (B), maintained. Determine K need from soil test results and use Table 6 manganese (Mn), and iron (Fe) to estimate K rate. Apply K in fall to established stands The need for application of Zn, Cu, B, Mn, or Fe has not when soil test K is below 200 ppm. For maintenance been demonstrated for peppermint in western Oregon. application (when soil test K is above 200 ppm), K can be applied in either spring or fall.

Table 6.—Fertilizer potassium (K) rate recommendations using the ammonium acetate extractant. Use this table for preplant soil sample from 0 to 6 inches and 0 to 2 inches in established stands. a Soil test K Apply this amount of K2O (ppm) (lb/a) Below 100 120–200 100–200 60–120 Over 200 None aAmmonium acetate extract method.

11 ­Conference Proceedings, Vol. 5, pp. 71–76. For more information March 6–7, 2003, Salt Lake City, UT. Evaluating Soil Nutrients and pH by Depth in Situations Christensen, N.W., J.M. Hart, M.E. Mellbye, and of Limited or No Tillage in Western Oregon , G.A. Gingrich. 2003. Sulfur influence on peppermint EM 9014-E. http://ir.library.oregonstate.edu/xmlui/ oil quality. In: Western Nutrient Management Confer- bitstream/handle/1957/19024/em9014.pdf ence Proceedings, Vol. 5, pp. 190–198. March 6–7, Fertilizer and Lime Materials , FS 52-E. http://extension. 2003, Salt Lake City, UT. oregonstate.edu/catalog/html/fg/fg52-e/ A Guide to Peppermint Insect and Mite Identification, Cook, G., J.M. Hart, N.W. Christensen, B. Quebbeman, PNW 182. http://ir.library.oregonstate.edu/jspui/ G. Kiemnec, and K. Diebel. 1999. Peppermint growth handle/1957/18341 and nutrient uptake in plants in the Grande Ronde Val- PNW Insect Management Handbook. http://uspest.org/ ley. MIRC Research Progress Report. pnw/insects Davis, E.A., J.L. Young, and S.L. Rose. 1984. Detection PNW Plant Disease Management Handbook. of high-phosphorus tolerant VAM-fungi colonizing http://plant-disease.ippc.orst.edu/ hops and peppermint. Plant and Soil 81:29–36. PNW Weed Management Handbook. http://uspest.org/ Feaga, J. and J. Selker. 2004. Field Measurements of pnw/weeds Nitrogen Leaching Below Willamette Valley Row Weed Management in Mint, EM 8774. Available from and Mint Crops. Oregon State University Extension OSU Crop Science, 541-737-8868. Service. EM 8851-E (available online and from OSU Department of Biological and Ecological Engineer- References ing). http://extension.oregonstate.edu/catalog/pdf/em/ Appleby, A.P. 2002. Department of Crop Science, Oregon em8851-e.pdf State University: Origin and Evolution, 1907–1990. Feaga, J., R. Dick, M. Louie, and J. Selker. 2004. Nitrates Baird, J.V. 1957. The influence of fertilizers on the pro- and Groundwater: Why Should We Be Concerned with duction and quality of peppermint in central Washing- Our Current Fertilizer Practices? Oregon State Univer- ton. Agron. J. 49:225–230. sity Extension Service. Special Report 1050. Brown, B. 1982. Nitrogen tissue test procedures for pep- Hart, J.M., B. Quebbeman, G. Cook, N.W. Christensen, permint. Proceedings of the 33rd Annual Northwest K. Diebel, and G. Kiemnec. 2000. The use and nutrient Fertilizer Conference, Boise, ID. content of peppermint slugs. Oregon Mint Commission Brown, B. 1983. Peppermint Tissue Analysis: Nitrogen. Update, Spring 2000. University of Idaho. Current Information Series 714. Hart, J.M., N.W. Christensen, M.E. Mellbye, and Brown, B., J.M. Hart, M.P. Wescott, and N.W. Chris- G.A. Gingrich. 2003. Nutrient and biomass accumu- tensen. 2003. The critical role of nutrient management lation of peppermint. In: Western Nutrient Manage- in mint production. Better Crops 87(4):9–11. ment Conference Proceedings, Vol. 5, pp. 63–70. March 6–7, 2003, Salt Lake City, UT. Christensen, N.W., J.M. Hart, M.E. Mellbye, and G.A. Gingrich. 1997. Optimal nitrogen and sulfur fer- Hart, J.M., N.W. Christensen, M.E. Mellbye, tilization of peppermint grown in the Willamette Val- G.A. ­Gingrich, and E. Marx. 1997. Nitrogen fertiliza- ley. In: 1996–97 Reports to the Agricultural Research tion of peppermint. Oregon State University Extension Foundation for the Oregon Mint Commission. Service. Crop and Soil News/Notes, 11(5):7–9. Christensen, N.W., J.M. Hart, M.E. Mellbye, and Hart, J.M. 1997. Nitrogen fertilization of peppermint. G.A. Gingrich. 1997. Impact of sulfur on peppermint Linn County Extension Association Update Newslet- oil quality. Oregon Mint Commission Update, Spring ter, 16(6):9. 1997. Hart, J.M. and T.L. Jackson. 1986. Peppermint rust Christensen, N.W., J.M. Hart, M.E. Mellbye, and ­control by desiccation with urea-sulfuric acid. th G.A. Gingrich. 1998. Optimal nitrogen fertilization pp. 173–178. In: Proceedings of 37 Annual Northwest of peppermint grown in the Willamette Valley. In: Fertilizer Conference, Boise, ID. 1997–98 Reports to the Agricultural Research Founda- Hee, S.M. 1975. The effect of nitrogen fertilization on tion for the Oregon Mint Commission. peppermint oil quality and content. M.S. Thesis. Christensen, N.W., J.M. Hart, M.E. Mellbye, and Oregon State University. G.A. Gingrich. 1998. Peppermint growth and nutri- Heutig, M.A. 1969. The effects of fertilizer treatments on ent uptake. Oregon Mint Commission Update, Spring oil content and nutrient concentration of peppermint in 1998. western Oregon. MS Thesis. Oregon State University. Christensen, N.W., J.M. Hart, M.E. Mellbye, and Jackson, T.L. 1972. The effect of phosphorus, potassium, G.A. Gingrich. 2003. Soil nitrogen dynamics in pep- and lime on the yield of peppermint oil in western permint fields. In: Western NutrientManagement ­ Oregon. Research Progress Report.

12 Jackson, T.L. and S.M. Hee. 1971. The effect of fertilizer N ­fertilization on peppermint yield and oil composi- treatments on the yield and quality of peppermint oil in tion. Washington State University. Washington Agri- Oregon. In: Proceedings, 22nd Annual Fertilizer Con- cultural Experiment Station Circular 541. ference of the Pacific Northwest. July 13–15, 1971, Schmitt, William G. 1996. The impact of weed popula- Bozeman, MT. tions on the production and quality of peppermint Kusonwiriyawong, C. 2005. Nitrogen mineralization from (Mentha piperita) oil. MS thesis. University of Wis- organic amendments during the second year following consin, Madison. application (peppermint hay evaluated as soil amend- Shizato, S. 2006. Decomposition and nitrogen transforma- ment). M.S. thesis, Oregon State University. tions of mint hay, composts, and cover crops incorpo- Landing, J.E. 1969. American Essence. A History of the rated into the soil. Oregon State University Department Peppermint and Spearmint Industry in the United of Crop and Soil Science. M.S. Environmental Soil States. A.M. Todd Co., Kalamazoo, MI. Science project report. McKellip, L. 1998. So You Want To Grow Better Mint? Smesrud, J.K. and J.S. Selker. 1998. Field sampling The Caxton Printers, Ltd., Caldwell, ID. considerations for the stem nitrate test in peppermint. Mitchell, A.R. 1996. Peppermint response to fertilizer in Comm. Soil Sci. Plant Anal. 29:3073–3091. an arid climate. J. Plant Nutrition 19:955–967. Wescott, M.P. and J.M. Wraith. 1995. Correlation of leaf Mitchell, A.R. and N.A. Farris. 1995. Peppermint chlorophyll readings and stem nitrate concentrations response to nitrogen fertilizer in central Oregon. in peppermint. Comm. Soil Sci. Plant Anal. 26:1481– In: Central Oregon Agricultural Research Center 1490. Annual Report, 1994. Special Report 941. Mitchell, A.R., N.A. Farris, and J.E. Light. 1995. New Acknowledgments tools for plant-response nitrogen testing of pepper- We appreciate careful and thoughtful review comments mint. In: Central Oregon Agricultural Research Center from T.J. Hafner, Wilbur-Ellis Company; Gregory C. Annual Report, 1994. Special Report 941. Biza, IP Callison & Sons; Rick Boydston, USDA-ARS, Nelson, C.E., R.E. Early, and M.A. Mortensen. 1971. Prosser, WA; Bradford Brown, University of Idaho; and Effects of growing method and rate and time of Bill Smith, William Smith Farms.

13 Appendix A Nutrient-related research in western Oregon peppermint

• 1955: N, P, K, S, and Mg were applied to a field near Jefferson in Marion County. Highest yield was achieved with an N rate of 150 lb/a. No other nutrient application increased yield, as soil test levels of other nutrients were adequate or high. • 1967: A survey to determine the nutrient status of pep- permint fields yielded the following results: — Micronutrient concentrations in plant tissue were relatively high. The highest tissue Mn concentra- tion was associated with lower oil production. — The highest N concentration was associated with rank, stemmy growth and decreased oil production. — Tissue P concentration was high, and no apparent Figure 14.—Research in 2005 measured effects of peppermint association existed between P concentration and oil residue application on plant-available nitrogen and soil nitrogen production. mineralization (see Appendix C, page 17). Photo: Gale Gingrich — The majority of the oil is produced in the top 15 inches of the plant. was relatively linear from late May to harvest, with 8,000 to 11,000 lb hay/a produced. Nitrogen uptake • 1968–1971: Experiments in grower fields in Benton, was 200 to 250 lb/a, and K uptake was 275 to 375 lb/a. Lane, Linn, and Josephine counties evaluated pepper- Peak N uptake of 2 to 3 lb/a/day occurred in mid-June mint hay production and oil yield response to appli- and was followed by peak biomass production of cation of N, P, K, and lime. Yield of peppermint hay 100 lb/a/day in late July. increased with increasing N rate up to 250 lb N/a, but decreased with higher N rates. Delayed N application • 1997 and 1998: A large-scale N rate evaluation was decreased hay yield. P application did not increase performed on a grower field near Junction City and tissue P concentration or oil concentration (percent oil on a second farm near Gervais. This effort confirmed extracted). In contrast, K application increased tissue earlier work suggesting that peppermint requires about K concentration but not oil concentration. Peppermint 200 lb N/a. Addition of more than 300 lb N/a did not stem nutrient concentration was evaluated as a man- increase hay or oil yield. The higher N rates increased residual soil NO agement tool for monitoring nutrient status during the 3-N after harvest. season. • 1998: The influence of S source and rate on oil qual- • 1980: Protocols for peppermint stem sampling were ity was evaluated. Excess SO4-S posed no threat to assessed at the University of Idaho Parma Research oil quality, while elemental S applied to foliage may Center. Subsequent stem nitrate monitoring projects at render peppermint oil unacceptable to buyers. OSU were based on the sampling protocols at Idaho. • 1996–2000 and 2001–2004: Variety trials were • 1985 and 1986: Liquid fertilizer material was evalu- conducted on fields infected with verticillium wilt at ated as a substitute for rust control by propane flaming the Botany and Plant Pathology Farm near Corvallis. in Benton, Lane, and Linn counties. Common liquid Results demonstrated that new experimental lines had fertilizer materials such as urea-ammonium nitrate improved wilt and rust tolerance, but lacked suitable (solution 32) did not adequately defoliate peppermint. oil quality. Treatments with a mixture of urea (15 percent N) and • 2003 and 2004: Plant-available N release from applied 49 percent sulfuric acid produced oil yield equal to peppermint residue was evaluated at the North Wil- treatments that received propane flaming. lamette Research and Extension Center, Aurora • 1993–1998: A long-term project evaluated N leaching (­Figure 14). Peppermint residue was applied in spring loss from vegetable and mint fields in Lane County, as 2003, and N fertilizer equivalency was evaluated for well as the use of stem nitrate-N as a tool for monitor- sweet corn crops grown in 2003 and 2004. Incubation ing crop N status. of soil from the NWREC trial was also performed. • 1995 and 1996: Biomass and nutrient accumulation • 2005: Mint stockpiles or “slugs” (after distillation) were measured in four grower fields. Rust was con- were sampled at four Willamette Valley farms from trolled chemically in two fields and by flaming in the August through October. Laboratory incubation was other two fields. Biomass accumulation in all fields performed to determine plant-available N release from peppermint residues.

14 Appendix B Effects of N application and maturity at harvest on oil quality

An overview of the influence of N application and Peppermint maturity at harvest increased oil yield maturity on peppermint oil quality is provided on page 8. (­Figure 19). Changes in oil component concentrations with N applic- Peppermint hay produced with 200 lb N/a and har- tion rate and maturity at harvest are shown here. vested at bud development state produced oil with 4.5 per- As N rate increased, oil quality changed as follows: cent menthyl acetate, below the desirable concentration. A similar N rate and harvest at 10 percent bloom should • Menthone concentration increased (Figure 15). increase menthyl acetate concentration to an acceptable • Menthol concentration decreased (Figure 16). level. Harvesting at this stage provided acceptable levels • Menthyl acetate concentration increased slightly with of menthone and menthol, as long as N rates were less N rate from 50 to 200 lb N/a (Figure 17). than 300 lb/a. Although menthyl acetate and menthone • Menthofuran concentration decreased (Figure 18). concentrations remained acceptable at 50 percent bloom, Because menthofuran increases with maturity, N harvesting at this stage is not recommended, as menthofu- indirectly influences menthofuran concentration by ran concentration increases to an unacceptable level after delaying maturity. 10 percent bloom. As maturity at harvest increased, oil quality changed as 8 Full bloom follows: • Menthone concentration decreased (Figure 15). 7 • Menthol concentration increased (Figure 16). 50% bloom 6 • Menthyl acetate concentration increased sharply Desired range (­Figure 17). • Menthofuran concentration increased (Figure 18). 5 Menthyl acetate (%) Bud 60 4 Bud 0 100 200 300 45 N rate (lb/a) Figure 17.—Menthyl acetate concentration of peppermint oil 50% bloom increases with maturity and is changed little by N rate. 30 8 Q Full bloom Menthone (%) u 15 a l Desired range Full bloom 6 i t 50% bloom 0 y 0 50 100 150 200 250 300 4 N rate (lb/a) Bud Figure 15.—Menthone concentration of peppermint oil increases 2 with N rate and decreases with maturity. Menthofuran (%) Desired range 60 0 Full bloom 0 100 200 300 N rate (lb/a) Q Figure 18.—Menthofuran concentration of peppermint oil increases 50 u 50% bloom a with maturity and decreases with N application rate when harvested l at 5075 percent bloom or later. i t y Desired range 60 Menthol (%) 40 45 Bud 30 30 Oil yield (lb/a) 15 0 100 200 300 N rate (lb/a) 0 Figure 16.—Menthol concentration of peppermint oil decreases Bud 50% bloom Full bloom with N application and increases with maturity. Figure 19.—Peppermint oil yield increases with maturity.

15 Appendix C Effects of weeds on peppermint oil quality

The presence of weeds in the field influences pep- consideration because of their impact on oil quality. permint oil yield and quality. Weeds reduce peppermint Other species, including those that are wind dispersed biomass through competition for resources, including and those that are rapidly developing resistance to sunlight, water, and nutrients. As weed biomass increases, commonly applied herbicides (e.g., marestail and a generally linear decrease in both peppermint hay and prickly lettuce), may become increasingly difficult to oil yield is expected (Figure 20). Weeds also can reduce control in the future. Some weed species (e.g., wild the life of the peppermint stand by forcing growers to garlic, volunteer garlic, and volunteer onion) must rotate to other crops in an effort to contain problem weed always be managed to limit their impact on oil qual- ­populations. ity. Conversely, low-density and sporadic populations Weeds present at peppermint harvest can also impart of grass weed species may not warrant the cost of undesirable color and odor to peppermint oil. Peppermint chemical management, given their lower impact on oil oil odor is one quality parameter. Oil quality based on quality. aroma testing is termed organoleptic evaluation. The pres- An understanding of the biology and ecology of ence of weeds in harvested peppermint can decrease the weed species can help growers time weed management positive organoleptic ratings and reduce the commercial practices to achieve maximum control. Specific weed value of the oil. For example, some weeds, such as mares- management recommendations are found in the mint tail, contain components that distill with the mint oil, chapter of the PNW Weed Management Handbook and resulting in off odors and flavors that reduce oil quality. In in OSU Extension publication EM 8774, Weed Man- extreme cases, oil may be commercially unacceptable. agement in Mint. Broadleaf weed species reduce oil quality more than do grass weed species. Unfortunately, broadleaf weed species ) are often more difficult to manage in peppermint than are 2 30 Hay yield 25 grass weed species. Perennial broadleaf weeds, including 25 20 Canada thistle and field bindweed, can be especially dif- 20 ficult to manage, since their life cycle is similar to that of 15 peppermint. 15 A trial conducted in Wisconsin quantified the impact 10 10 on oil quality of three weed species growing at a range of Oil yield densities in peppermint (Figure 21). As the weed biomass 5 5 increased relative to harvested peppermint biomass, oil Peppermint oil yield (lb/a) Peppermint hay yield (lb/100 ft 0 0 quality decreased. The three weed species varied in their 0 5 10 14 impact on oil quality. For example, at similar biomass Weed biomass (lb/100 ft2) levels (greater than 2.5 percent of harvested biomass), the Figure 20.—Characteristic yield loss for peppermint grown in broadleaf weed species (redroot pigweed and common competition with a mixed population of grass and broadleaf weed lambsquarters) decreased oil quality more than did barn- species. yardgrass, a grass species. Management of both grass and broadleaf weeds is criti- Barnyardgrass cal for peppermint yield and oil quality. Nonetheless, costs of weed management must be weighed against potential gains in oil yield and quality. These costs include direct herbicide and application costs, indirect costs (includ- ing peppermint injury and potential stand loss resulting from some herbicide applications), and development of Redroot herbicide-resistant weed populations due to overuse of pigweed Common certain herbicides. Individual growers should make weed Organoleptic quality lambsquarters management decisions on a field-by-field basis. Weigh all potential benefits and costs associated with the particular operation and cropping systems. 0 2.5 5 10 Some weeds may warrant greater weed management Weed biomass as a % of harvested biomass efforts than others. Broadleaf weeds such as common Figure 21.—Influence of weed biomass on organoleptic rating. groundsel and the pigweed species may require special Source: Adapted from Schmitt (1996).

16 Appendix D Plant-available N release from peppermint residue

Peppermint residue consists of the cooked leaves and stems of the peppermint plant after high-temperature distillation to extract oil. This residue has value as a soil amendment. It is often applied to fallow fields in the fall or sold as a soil amendment to gardeners. Research was conducted to determine the plant-available N (PAN) sup- plied by peppermint residue.

Peppermint residue N concentration Willamette Valley peppermint residue stockpiles (also called “slugs” or peppermint hay) were sampled follow- ing harvest. Three replicates of peppermint slugs from four farms (24 samples) were collected during August and October 2005 (Table 7). Both N concentration and C:N ratio were relatively consistent in all samples, despite differences in storage time in the field (1 to 8 weeks), Figure 22.—Decomposition of peppermint residue in laboratory moisture, and observed fungal growth in the piles. incubation. Soil was a Chehalis silty clay loam, 25 percent gravi- metric moisture. Source: Shizato (2006). Short-term effects on PAN Laboratory incubation with moist soil at 72°F simu- lated the rapid decomposition that occurs when pep- permint residue is tilled into moist soil (Figure 22). The laboratory residue incorporation rate was roughly equiva- lent to a field application rate of 1 to 2 inches of residue tilled into a 6-inch soil depth (roughly 100 wet ton pep- permint residue/a). Peppermint residue lost about one-third of its organic matter via decomposition during the first 30 days of lab incubation. During this period, soil nitrate-N was immo- bilized (reduced) by microbial activity (Figure 23). This effect is typical of plant materials with relatively low total N (1.3 to 2.2 percent N, dry wt basis) and high C:N ratios (above 15). Consumption of nitrate-N by microbes was temporary, Figure 23.—Immobilization of plant-available N during initial however. After 30 days, decomposition slowed, and soil N decomposition of peppermint residue in laboratory incubation. Soil was converted to nitrate-N at about the same rate in both was a Chehalis silty clay loam, 25 percent gravimetric moisture. peppermint-amended and unamended soil (Figure 23). Source: Shizato (2006). These results suggest that peppermint residue can be incorporated into soil in the fall without risk of nitrate leaching to groundwater, as the initial decomposition process will consume soil nitrate-N.

Table 7.—Peppermint residue characteristics.a Dry matter Carbon (C) Total nitrogen (N) (% of wet weight) (% in dry matter) (% in dry matter) C:N ratio Range 37–65 39–42 1.3–2.2 17–32 Average 50 40 1.8 22 aOn-farm stockpiles at four Willamette Valley farms collected August and October, 2005. Source: Peppermint hay samples collected by John P.G. McQueen. Incubation data reported in Shinzato (2005).

17 N fertilizer replacement value of peppermint residue The N fertilizer replacement value of peppermint residue for a sweet corn crop was determined in field trials at the North Willamette Experiment Station in 2003 and 2004. Peppermint residue from the 2002 harvest was incorporated before planting sweet corn in May 2003. Residue was applied at a rate of 9 dry ton/a and supplied 620 lb total N/a. The N fertilizer replacement value of peppermint residue (relative to urea) was 40 lb N/a in 2003 and 30 lb N/a in 2004 (Kusonwiriyawong 2005).

Effects on long-term soil N mineralization rate After 2 years of sweet corn production, soil from the 0- to 6-inch depth was sampled in May 2005 and incubated in the laboratory (Figure 24). The incubation Figure 24.—Peppermint residue effect on long-term soil N miner- ­demonstrated the following: alization rate. Peppermint residue was applied in spring 2003, and • The rate of PAN release was consistent (linear) under soil incubation data were collected from a spring 2005 soil sample. lab conditions (moist soil, 72°F) for a long incubation Soil was a Willamette silt loam, 25 percent gravimetric moisture, period. NWREC, Aurora, OR. Source: Sullivan and Beldin (unpublished data). • A one-time peppermint application (spring 2003), performed 2 years before soil sampling (spring 2005), roughly doubled the soil N mineralization rate. (The rate increased from 0.35 to 0.6 ppm N/day.) Because of long-term effects on N mineralization, pep- permint residues should be applied evenly at a moderate application rate. Do not apply peppermint residues to the same fields every year.

This publication replaces FG 15-E, Peppermint: West of Cascades Fertilizer Guide.

© 2010 Oregon State University. This publication may be photocopied or reprinted in its entirety for noncommercial purposes. This publication was produced and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregon counties. Oregon State University Extension Service offers educational programs, activities, and materials without discrimination based on age, color, disability, gender identity or expression, marital status, national origin, race, religion, sex, sexual orientation, or veteran’s status. Oregon State University Extension Service is an Equal Opportunity Employer. Published December 2010. 18